Hi,

I'm developing my own TCP stack and I'm having a problem with
calculating the TCP checksum. According to the information I gathered
from the internet, the TCP checksum is calculated based on the IP
pseudoheader (that is source IP, destination IP, protocol (0x06 for
TCP) and overall packet length) and complete TCP header together with
data to be sent. The checksum is one's complement of sum of all 16-bit
words. I'm almost sure it is calculated correctly as it works ok for
the IP header. Do you have any suggestions on what can I be doing
wrong? Do I have to convert all header data to network byte order
before calculating the checksum (I'm doing it this way now)?

Thanks in advance

Chris

In article <1192353288.702518.237280@q3g2000prf.googlegroups. com>,
autom4gician@gmail.com wrote:

> Hi,
>
> I'm developing my own TCP stack and I'm having a problem with
> calculating the TCP checksum. According to the information I gathered
> from the internet, the TCP checksum is calculated based on the IP
> pseudoheader (that is source IP, destination IP, protocol (0x06 for
> TCP) and overall packet length) and complete TCP header together with
> data to be sent. The checksum is one's complement of sum of all 16-bit
> words. I'm almost sure it is calculated correctly as it works ok for
> the IP header. Do you have any suggestions on what can I be doing
> wrong? Do I have to convert all header data to network byte order
> before calculating the checksum (I'm doing it this way now)?

Have you read RFC 1071, "Computing the Internet Checksum"?

--
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
*** PLEASE don't copy me on replies, I'll read them in the group ***

On 14 Pa , 19:20, Barry Margolin <bar...@alum.mit.edu> wrote:
> In article <1192353288.702518.237...@q3g2000prf.googlegroups. com>,
>
> autom4gic...@gmail.com wrote:
> > Hi,
>
> > I'm developing my own TCP stack and I'm having a problem with
> > calculating the TCP checksum. According to the information I gathered
> > from the internet, the TCP checksum is calculated based on the IP
> > pseudoheader (that is source IP, destination IP, protocol (0x06 for
> > TCP) and overall packet length) and complete TCP header together with
> > data to be sent. The checksum is one's complement of sum of all 16-bit
> > words. I'm almost sure it is calculated correctly as it works ok for
> > the IP header. Do you have any suggestions on what can I be doing
> > wrong? Do I have to convert all header data to network byte order
> > before calculating the checksum (I'm doing it this way now)?
>
> Have you read RFC 1071, "Computing the Internet Checksum"?
>
> --
> Barry Margolin, bar...@alum.mit.edu
> Arlington, MA
> *** PLEASE post questions in newsgroups, not directly to me ***
> *** PLEASE don't copy me on replies, I'll read them in the group ***

I read that RFC, and my algorithm was correct. The problem was with
the data - I failed to convert one of the header fields to network
byte order. I managed to solve my problem, but thanks anyway for your
. A calculator seems the best tool in cases like this

Chris